Superconducting films on alkaline earth fluoride substrate with multiple buffer layers

ABSTRACT

A high frequency superconducting device structure is disclosed which comprises an alkaline earth fluoride substrate with a magnesium oxide lower buffer layer on the alkaline earth substrate and an upper buffer layer epitaxial template layer on the magnesium oxide layer for providing a template for epitaxial growth of a high temperature superconducting film on the upper buffer layer, providing reduced dielectric and conducting losses at high frequencies. The disclosed structure may be incorporated into a multi-chip module (MCM) for providing very high speed interconnections.

This is a continuation of application Ser. No. 08/030,733 filed on 12Mar. 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to superconducting structures and theirfabrication. In particular, this invention relates to low losssuperconducting films on alkaline earth fluoride substrates.

The invention also generally relates to metal oxide structures. Moreparticularly, this invention relates to a metal oxide film grown on asingle crystalline silicon substrate with a buried oxide layer, and tofree standing films formed therefrom.

This invention also generally relates to a substrate fixture assemblywhich is particularly useful for pulsed laser deposition. Moreparticularly this invention relates to a substrate fixture providingsimultaneous double sided coating and simultaneous multiple substratecoating.

2. Discussion of the Background

Low loss superconductive components can make vital contributions tocurrent and future microwave and millimeter wave systems. There havebeen significant efforts to develop high temperature superconductingthin films (HTSC) for applications in the micro-wave components. TheHTSC thin films used in typical micro-wave components must be depositedon a micro-wave compatible substrate having a low dielectric constant(ε) and a low loss tangent in order to avoid unacceptable powerdissipation in the substrate.

Of all the metal-oxide superconductors, thin films of Y₁ Ba₂ Cu₃ O_(7-x)for x=O.5 to 1.0 (YBCO) are the most technologically advanced. State ofthe art YBCO films on LaAlO₃ substrates have achieved the bestmicro-wave performance. For example, pulsed laser deposition (PLD)deposited and patterned YBCO films on LaAlO₃ have shown surfaceresistance values lower than that of Cu by a factor of 20 at X-band(X-band spans 8-12 Ghz) and lower than that of Cu by more than twoorders of magnitude at L-band (below 5 Ghz). However, LaAlO₃ has adielectric constant (ε) of 25.

While a variety of applications at the lower end of the micro-wavespectrum could be realized with current HTSC technology, satellitecommunications, radars and missile seekers all require higherfrequencies in the millimeter wavelength region. In order forsuperconductor films to be useful at those higher frequencies substratedielectric losses must be reduced, which requires a substrate with a lowdielectric constant.

At high frequencies superconductors are not completely lossless. When asuperconductor is used as a high frequency line to transport a signal,the resistive loss associated with that line decreases as the line widthincreases. Therefore, it is desirable to have relatively wide lines inorder to reduce resistive loss. However, dielectric loss is proportionalto the dielectric constant of a material and to the volume of dielectricmaterial through which a high frequency electromagnetic wave istransported. Therefore, when the width of a superconducting line on adielectric substrate is increased, the dielectric loss is alsoincreased. Therefore it is useful to lower the dielectric constant ofthe dielectric material in which a superconducting line is formed inorder to achieve to a superconducting waveguide which has the lowestpossible high frequency loss.

A report prepared for the GE Astro Space Division by Belohoubek et al(Dec. 89) on "Applications of high T_(c) superconductors for satellitecommunication systems" points out that even at the C-band frequencies(3.6-6.2 gigahertz) a low dielectric constant is important for asubstrate because substrate dielectric loss becomes important for lightweight, high performance filters. The relatively large dielectricconstants of LaAlO₃ (ε=25), Sapphire (ε=11) and MgO (ε=9.5) imposerestrictions on the possible use of those substrates in conjunction withthe high quality YBCO films, at least at millimeter wave frequencies. Bya high quality HTSC film is meant an HTSC film which has a surfaceresistance at 10 gigahertz of less than 10 milli-ohms and preferablyless than 1 milli-ohm.

Alkaline earth fluorides with the general formula MF₂ (where M=Mg, Ba,Sr, Ca) have static dielectric constants between 5 and 7 at liquidnitrogen temperatures (77K) and have been used as substrate material forYBCO films. However, YBCO films grown on MF₂ substrates have poorsuperconducting properties, such as the sharpness of the normal tosuperconducting transition and resistance versus temperature, asreported by P. Madakson et al. in Journal of Applied Physics, volume 63,No. 6, pp. 2046 (1988), and Siu-Wai Chan et al. in Applied PhysicsLetters, Volume 54, No. 20, pp 2032 (1989). Those YBCO films are alsopolycrystalline thereby introducing additional energy loss mechanisms athigh frequency. Therefore those films are not suitable for micro-waveand millimeter-wave applications.

There have also been attempts to deposit YBCO films on top of a CaF₂buffer layer buffering a GaAs substrate, as reported by K. Mizuno et alin Applied Physics Letters, Vol. 54, No. 4, pp. 383, (1989). The qualityof those YBCO films is rather poor relative to the film quality whenYBCO is deposited on high quality substrates such as LaAlO₃. There is acontinuing need for superconducting thin films on substrates whichprovide lower energy loss at high frequencies than is now possible.

Another need is for improved infra-red (IR) detectors. It is known thatsuperconducting films may be used for IR detectors. Low heat capacitysubstrates which have high thermal conductivity are desired forsuperconducting IR detectors in order to increase detector signal andspeed. One substrate which has very high thermal conductivity isdiamond. However high quality high T_(c) films can not be deposited ondiamond substrates due to lattice and thermal expansion mismatch and dueto reaction of diamond with oxygen.

HTSC films have been deposited upon buffered reactive substrates bydepositing a buffer layer on the reactive substrate prior to depositingan HTSC film. By a reactive substrate we mean any substrate upon which athin film of a HTSC substantially interacts with the substrate so thatthe HTSC film is not superconducting or does not have a superconductingtransition temperature or as high a critical current density withinabout a hundred angstroms of the interface between the HTSC film and thesubstrate.

In all the deposition schemes, there remains the difficulty of preparinga superconducting film which does not degrade due to its substrate and asubstrate whose properties do not detract from the desired function of asuperconducting device into which it is incorporated.

Many thin films have been deposited by PLD. During PLD, pulses of anultraviolet laser are directed onto a target material. Each pulsevaporizes a small amount of the target surface. The vaporized materialinteracts with both the laser beam and target surface to create a denseplume of material rapidly travelling directly away from the target. Acenter of momentum of the plume is directed along the target surfacenormal so that a substrate surface is usually positioned opposing thetarget surface in order to block the plume. The plume impinges upon thesubstrate and deposits thereon to form a film. The material in the plumetravels mainly directly away from the surface along the target surfacenormal. The plume also usually contains particles on the order of 1000angstroms to 10 micron.

HTSCs such as YBa₂ Cu₃ O₇ are typically deposited onto singlecrystalline substrates held at a temperature between 600° and 850° C.Many applications require HTSC films deposited with a substrate surfacetemperature uniformity of ±5° C. for optimal growth of the desiredphase. For single-sided coatings of substrates of diameter of less than1 inch, substrates have been adhered to a hot plate with a high thermalconductivity paste in order to provide the desired temperature control.Radiative heaters have been used to heat the back sides of largersubstrates, i.e., the side of the substrates that faces away from thetarget.

The large particles ejected from a target during PLD impinge upon andremain on the substrate. For many applications, those particulates arecatastrophic. Also, for many technologically important applications, itis necessary to deposit buffer layers or create multilayer structures,which are degraded by the large particles in PLD films. Presentsubstrate fixtures for PLD allow only single sided coating of substratesand are only suitable for coating a single substrate at a time. Whilesuch substrates are acceptable for research, they severely limit thecommercial applications because of low throughput and the restriction ofsingle side coating. Clearly PLD systems have several disadvantageslimiting their usefulness for manufacturing thin film devices. A needexists to overcome all those disadvantages if PLD is to ever becomecommercially important.

Herein, perovskite crystal structure type material means any materialwhich has the CaTiO₃ cubic crystal structure type or any of thetetragonal and rhombohedral or orthorhombic distortions of that cubiccrystal structure type.

Herein, by epitaxial is generally meant that a layer grown on a singlecrystal substrate surface is also single crystal and has a particularcrystallographic relationship to the substrate surface and that thatcrystallographic relationship is determined by the crystallography ofthe substrate surface and the material and deposition conditions of thegrown layer. Particular crystallographic relationships either indicatingalignment of crystallographic axes of a substrate with crystallographicaxes of a film or alignment of crystallographic axes of substrate andfilm and also constraint of film lattice constants to be the same aslattice constants of the substrate at the film/substrate interface aredisclosed for various interfaces throughout this application.

As indicated by the foregoing discussions, high temperaturesuperconducting film technology has not progressed to a point enablinglarge scale integration of multiple functions on a single substrate.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide asuperconducting thin film on a substrate which has superior microwaveproperties.

Another object of this invention is to provide an oxide superconductorthin film epitaxially grown on a MF₂ substrate which has reducedmicrowave loss.

Another object of the invention is to provide a free standingmetal-oxide thin film having superior properties such as thicknessuniformity, single crystal quality, and reduced thickness.

Another object of the invention is to provide a freestanding film of asuperconducting material having the aforementioned properties.

Another object of the invention is to provide a free standing filmbonded onto a different substrate from the substrate upon which it wasgrown.

Another object of the invention is to provide a novel substrate fixturefor simultaneously heating multiple substrates and for allowingsimultaneous coating on both sides of those substrates.

Another object of the invention is to provide a novel substrate fixturewhich provides coating uniformity by use of a flow through gas supply.

The present invention provides a novel high frequency device structure,comprising a single crystal (001) oriented MF₂ substrate, an oriented(100) MgO buffer layer on the substrate, a second buffer layer on theMgO layer and upon which high quality HTSC films can be grown, and ahigh quality oxide superconducting layer on and contacting the secondbuffer layer, and a process for forming that device structure.Preferably, the superconducting film is YBa₂ Cu₃ O_(7-x) with x nearzero and the substrate is magnesium fluoride.

The present invention provides a novel process for forming a novel freestanding film by separating a film from a supporting structure, saidstructure and film comprising a silicon substrate with a buried oxidelayer, buffer layers on the substrate, and a metal-oxide layer on thebuffer layers, comprising forming said structure and selectively etchingaway at least the silicon substrate.

The present invention provides a novel substrate fixture comprising anenclosure having an enclosure first major surface opposing an enclosuresecond major surface, a substrate holder means for holding a substrateso that a flat substrate first major surface is opposing one of theenclosure first and second major surfaces and is between the enclosurefirst and second major surfaces, means for rotating the substrate holdermeans about an axis parallel to the normal to the substrate first majorsurface. The enclosure has an opening disposed to overlap a planedefined by the substrate first major surface. Preferably, the enclosureincludes a heater to uniformly heat the substrate. Preferably, theholder means has means to simultaneously hold several parallel andclosely spaced substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a first embodiment of the invention;

FIG. 1A shows a sectional view of a variation of the first embodiment ofthe invention;

FIG. 2 shows a sectional view of a second embodiment;

FIG. 3 shows the thin film of the second embodiment detached from itsoriginal substrate;

FIG. 4 shows a sectional view of the thin film of the second embodimentin which that film has been attached to a new substrate;

FIG. 5 shows a sectional overview of a third embodiment of the presentinvention;

FIG. 5A shows a sectional overview of a variation of the thirdembodiment of the present invention;

FIG. 5B shows a sectional overview of another variation of the thirdembodiment of the present invention;

FIG. 6 shows a more detailed sectional view of the third embodiment;

FIGS. 7A-7C show various views of a substrate magazine of the thirdembodiment;

FIGS. 8A-8C show test fixtures and substrate data for examples 8A-8C,respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIRST EMBODIMENT

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein FIG. 1shows a layered structure A having substrate 1 of (001) oriented MgF₂.MgF₂ has a dielectric constant of 5.2 at 77K. Epitaxial MgO buffer layer2, is deposited directly upon substrate 1 and is oriented (100). Thinfilm upper buffer layer 3, preferably of (100) oriented NdGaO₃, is grownon MgO buffer layer 2. Superconducting thin film 4 is grown on upperbuffer layer 3.

Substrate 1 may be any alkaline earth fluoride substrate. Preferably,superconducting film 4 is YBCO, but it may be any of the HTSCs such asBiSrCaCuO and TlBaCaCuO superconductors and their analogues. Analoguesmeans any of the materials having the same crystal structure as that ofknown superconducting compounds consisting of the sets of elementslisted above, but in which at least one element has been partially ortotally substituted by another chemically similar element and thatchemically similar element resides at the lattice site of the removedelement. Analogues also means those compounds in which certain of theelements listed in the sets of elements mentioned above have zeroconcentration, such as TlBaCuO compounds which have zero Ca content. Theinvention is not limited to any particular one of those HTSCS. However,higher quality films of YBCO than of the other known HTSCs can befabricated at this time. YBCO analogues include compositions in which Yis substituted by any of the rare earths that are well known to formsuperconducting materials and/or lanthanum.

MgF₂, and the other alkaline earth fluorides are very chemicallyreactive with oxides at the growth temperatures and high oxygen partialpressures necessary for deposition of oxide high temperaturesuperconductors. MgO buffer layer 2 is very unreactive and functions tochemically isolate substrate 1 from upper layers of layered structure A.

Single crystalline (100) oriented MgO thin films can be grown onalkaline earth fluoride substrates. Low growth temperature ensures thatno MgO/substrate interdiffusion occurs during growth.

MgO buffer layer 2 is preferably grown at lower temperatures of lessthan 600° C. At temperatures of less than 600° C. the MgF₂ does notdiffuse into MgO buffer layer 2 and MgO buffer layer 2 does not diffuseinto the MgF₂. Mgo buffer layer 2 both isolates subsequent layers fromsubstrate 1 and grows epitaxially on substrate 1 so that a (100) surfaceis presented to upper buffer layer 3.

Preferably, MgO buffer layer 2 is at least 30 angstroms thick so that itcompletely isolates substrate 1 from layers deposited upon Mgo bufferlayer 2. Preferably, MgO buffer layer 2 is no more than one micron thickso that the dielectric loss properties of the MgO do not becomeconsiderable.

Upper buffer layer 3 is grown directly on MgO buffer layer 2 andfunctions to provide an epitaxial lattice match for growth ofsuperconducting layer 4 so that superconducting layer 4 growsepitaxially upon upper buffer layer 3 and with minimal stress andminimal defects. Alternatively, upper buffer layer 3 may be grown on anintermediate buffer layer disposed between the MgO and upper bufferlayer 3 in order to provide additional strain relief or latticematching.

In a preferred embodiment, upper buffer layer 3 comprises a perovskitecrystal structure type material, such as NdGao₃, LaGaO₃, LaAlO₃, orSrTiO₃. Preferably, upper buffer layer 3 is greater than 50 angstromsthick, because that thickness allows lattice mismatch at the interfacebetween the MgO buffer layer 2 and upper buffer layer 3 to not affectthe upper surface of upper buffer layer 2, so that growth ofsuperconducting film 4 is not affected by strain at the interfacebetween the two buffer layers.

Preferably, the lattice mismatch between superconducting film 4 andupper buffer layer 3 is less than 6 percent. Lattice mismatch of lessthan 6 percent provides for relatively high quality HTSC films. Forexample, the lattice mismatch between the average of the a and b latticeconstants of YBCO and the lattice constant of all four of theperovskites mentioned above is less than 3 percent. The a and b latticeconstants of YBCO are 3.82 and 3.89 angstroms which are very close toone another. While perovskites are not always exactly cubic thevariation of their lattice constants is usually very small and they areusually considered to be cubic and to have a single average latticeconstant value, at least when used for HTSC substrates.

In a preferred embodiment upper buffer layer 3 comprises NdGaO₃ andsuperconducting film 4 comprises YBCO, because of the minimal latticemismatch of less than 1 percent between the a and b axes of YBCO and thelattice constant of 3.83 angstroms of NdGaO₃. In this instance byepitaxial is meant that the a and b axes of superconducting film 4 alignat the interface with cubic axes of the NdGaO₃ and that the latticeconstants of superconducting film 4 at the interface with NdGaO₃ areconstrained to the NdGaO₃ lattice constant. Defects in the interfacestructure of superconducting film 4 may occur to reduce stress.

Upper buffer layer 3 may also comprise LaSrGaO₄, NdCaAlO₄, or LaSrAlO₄because those materials grow epitaxially on MgO and high quality HTSCfilms can be grown upon surfaces of those materials. In particular,epitaxial YBCO films can be grown upon those substrates. By epitaxial inthis instance is meant that the YBCO films are oriented c axis out ofthe plane and have their a and b axes aligned with the a and b axes ofupper buffer layer 3 or alternatively that the YBCO films are orientedc-axis in the plane of the film and have either their a or their b axisout of the plane.

Upper buffer layer 3 may also be CeO₂ which is cubic or yttriastabilized zirconia (YSZ) which is also cubic. YSZ contains about 9 molepercent of Y₂ O₃ to stabilize the cubic phase. High quality epitaxialYBCO can be grown on those substrates. In this instance by epitaxial ismeant that the YBCO has its c axes out of the plane and has its a and baxes in the plane, but rotated at 45 degrees relative to the a and baxes of upper buffer layer 3 or that the YBCO film is oriented c-axis inthe plane with either its a or its b axes out of the plane and in whichcase the YBCO film c-axis in the plane and the a or b axis in the planeare canted at 45 degrees relative to the cubic axes of, in this case,cubic upper buffer layer 3.

Effective surface resistance, which is defined to be a thin film surfaceresistance value obtained assuming both substrate dielectric loss andsuperconducting film resistive loss are due only to film surfaceresistance, for layered structure A, has the same frequency dependenceas the best oxide high temperature superconducting films so far obtainedon any substrate when superconducting layer 4 is YBCO.

A large lattice mismatch of 8.8% exists between MgO (a 4.21 Å) and YBCO(a=3.82 Å, b=3.89 Å), and films of YBCO grown on MgO exhibit apolycrystalline mosaic microstructure. The orientational relationshipbetween an MgO substrate and a YBCO film are established to be [110]YBCO parallel to (1001 MgO in the (a-b) plane of the film and (001) YBCOparallel to [001] MgO along the c-axis of the film. Further, in theresulting mosaic microstructure, the in-plane orientational relationshipis different from one grain to another. The absence of complete a-bplane locking results in a c-axis oriented polycrystalline film in whichthe grain boundaries act as weak-links resulting in microwave losses.

YBCO films grown on substrates such as SrTiO₃ with a closer latticematch (a=3.9 Å) on the other hand exhibit a single crystallinemicrostructure with epitaxial relationships such as [100] YBCO parallelto [100] SrTiO₃ in the (a-b) plane of the film and [001] YBCO parallelto [001] SrTiO₃ along the c-axis of the film. SrTiO₃ used as upperbuffer layer 3 mainly provides an ideal epitaxial structural templateover the (100) MgO buffer layer 2, for growth of superconducting film 4of YBCO.

MgF₂ is chemically reactive at the high growth temperatures (>750° C.)and high oxygen partial pressures (on the order of 250 mTorr) needed toobtain high quality YBCO films. Though it is possible to grow YBCO filmsdirectly on MgF₂ substrates at relatively lower temperatures in thevicinity of 650° C., the superconducting properties of films grown atthose low temperatures are rather poor (with the transition temperaturesaround 80-83K). Even at the lower growth temperatures deposition offilms on magnesium fluoride substrates lead to film-substrate reactionswhich poison properties of both the film and the MgF₂ substrate. Similarfilm-substrate reactions occur when YBCO films are grown on otheralkaline earth fluoride substrates.

EXAMPLE 1

Prior to fabrication, the deposition vacuum chamber was pumped down tovacuum levels of the order of 104 Torr and then raised throughintroduction of oxygen in order to maintain the required oxygen partialpressure during deposition. All the three layers were deposited in asingle pump-down cycle, without exposing the chamber to the ambientatmosphere between each layer. The targets were held on a carrousel ofthe sort described by Chase et al. in "Multilayer high T_(c) filmstructures fabricated by pulsed laser deposition of Y--Ba--Cu--O",Journal of Materials Research, volume 4, 1989, pp. 1326-1329. During thedepositions, the carrousel would rotate the chosen target about thetarget axis with the laser beam hitting the circumference of the target.

Between the depositions, the carrousel itself would rotate about its ownaxis to place the required target in the laser beam.

PLD was used to deposit an MgO buffer layer, a SrTiO₃ buffer layer and aYBCO film onto an MgF₂ substrate. PLD is well known. See for example,Singh et al. in "Theoretical model for deposition of superconductingthin films using pulsed laser evaporation technique", Journal of AppliedPhysics, volume 68, 1990, pp 233-246. A KrF excimer laser deliveredoptical pulses of 248 nm radiation with pulse widths of 20 ns, pulseenergy densities of 1.5 J/cm² on stoichiometric targets, at pulserepetition rates of 10 Hz. A substrate holder/heater holding thesubstrate maintained the substrate at a temperature of 700° C. duringdeposition of YBCO and SrTiO₃ buffer layers, and at 600° duringdeposition of the MgO buffer layer.

The substrate onto which the films were deposited was 10 mm×10 mm and0.5 mm thick, and is commercially available from Solon Technologies Inc.of Solon, Ohio.

The MgO layer was deposited to a thickness of 1000 Å in an oxygenpartial pressure of 1 Mtorr at 600° C. The MgO target is commerciallyavailable from Canadian Substrate Supplies Ltd. of Niagara Falls, N.Y.

A 100 Å thick buffer layer of SrTiO₃ was deposited in an oxygen partialpressures of 250 mTorr onto a substrate maintained at 700° C. The targetfor deposition of the (100) SrTiO₃ buffer layer was a single crystal ofstoichiometric composition from commercial Crystal Laboratories, Inc. ofNaples, Fla.

The YBCO layer was deposited to a thickness of about 3000 Å. The targetfor the laser deposition of the YBCO layer was a pellet of sinteredpowder of YBCO and is available from Seattle Specialty Ceramics ofWoodinville, Wash. After the depositions the film was cooled to roomtemperature at the rate of 75° C./min in 200 Torr of oxygen.

The superconducting transition temperature of the YBCO film wasdetermined by ac susceptibility and dc resistivity measurements carriedout as a function of temperature. The YBCO film of example 1 of theinvention exhibited a superconducting transition temperature of 90K witha transition width less than 1K.

The critical current density of the YBCO film of example 1, J_(c) atwhich superconductivity deteriorates (defined as the current density atwhich a voltage drop of 1 mV/mm is observed along the conductiondirection) was estimated by I-V measurements using a 100 mm-wide and 2mm long bridges. At 77K, and zero applied magnetic field, J_(c) was4×10⁶ A/cm².

The crystalline quality of the film-buffer layer of Example 1 wasdetermined by x-ray diffraction. The diffraction shows a high degree ofcrystalline quality and indicates that the c-axis of the YBCO film, bothbuffer layers, and the substrate all aligned parallel to each other,indicating a high degree of epitaxial growth.

The micro-wave/millimeter-wave properties of the YBCO film of Example 1were measured using a dielectric resonator at 24 Ghz in which the topsurface of the resonator was formed by the superconducting film. Thefilms have unloaded Q values in excess of 30,000. That value indicatesthat the top surface resistance of films of example 1 are as low as thehighest quality HTSC superconducting films.

COMPARATIVE EXAMPLE 1

A YBCO film was grown on the (001) MgF₂ substrate without any bufferlayer at temperatures and pressures typically used as mentioned above.The film is non-superconducting at and above 77K.

COMPARATIVE EXAMPLE 2

A YBCO film was grown on a (001) MgF₂ substrate with a (100) MgO bufferlayer. The film has a superconducting transition temperature of 83K. Thetransition width is about 2K. The structural quality of the film asstudied by x-ray diffraction is rather poor (indicating polycrystallinematerial) and is therefore not suitable for micro-wave andmillimeter-wave components.

YBCO film 4 grown on a (001) MgF₂ substrate with a first buffer layer of(100) MgO and a second buffer layer of (100) SrTiO₃ exhibits goodcrystallographic structure and excellent superconducting propertiesessential for applications in the micro-wave and millimeter-wavefrequencies. The upper buffer layer promotes highly epitaxial YBCO filmgrowth, and promotes growth of a YBCO film which does not have grainboundaries.

Multilayer structure A may be used as part of a high frequency resonatorby forming a wall of the resonator from the superconducting film ofmultilayer A. Multilayer structure A may also be used as a transmissionline for high frequency.

Although the invention has been described with reference to YBCO, almostall of the HTSC oxide superconductors have crystal structures with verysimilar a and b lattice constants so that they grow with the highestfilm quality and with the lowest surface resistance on the samesubstrates upon which YBCO grows best. Therefore this invention can beused with any of those HTSCs.

Although pulsed laser deposition was used in the disclosed examples, theinvention may be practiced with all other deposition techniques, e.g.,magnetron sputtering, or e-beam deposition.

Although the invention has been described with reference to (001) MgF₂substrate, any of the alkaline fluoride materials may be used as thesubstrate. MgO is not reactive with the other alkaline earth fluoridesand may be deposited upon those alkaline earth fluorides at lowtemperature.

The invention thus provides a superconducting layered structure whichhas superior superconducting properties and a low dielectric constantand low dielectric loss substrate. Therefore it can be advantageouslyused in low loss superconducting components in the microwave andmillimeter-wave frequencies.

The invention described above is useful in multi-chip modules (MCMs)because it provides for very high band width and low dispersioninterconnection lines between integrated circuit (IC) chips of a MCM. AnMCM is defined to be a rigid structural member for structurallysupporting a set of IC chips and which comprises conductinginterconnection lines for electrically connecting the active electricalfunctions of those IC's so that they work together to provide intendedfunctions. Superconducting film 4 provides a layer which may bepatterned into signal lines for connecting the ICs of an MCM. Inclusionof a ground plane in substrate 1 then provides a very high band widthtransmission line comprising the ground plane and the signal linesformed from superconducting film 4 for interconnection of ICs of an MCM.As further discussed below, that ground plane may also besuperconducting and multiple layers of signal lines may be present.

In FIG. 1 only a single superconducting layer 4 is disclosed. However,the invention embodied in FIG. 1 is also advantageous in devices inwhich there are a series of high quality superconducting layers, atleast one of which is seperated from other conducting layers by a lowdielectric loss insulator. Those superconducting layers may be used toprovide power through a power supply layer, provide a ground planethrough a ground plane layer for acting as the ground plane of a veryhigh band width transmission line for a signal line layer fortransporting signals at high frequencies.

There may be more than one superconducting signal line layer and morethan one ground plane layer. The signal line layers may be sandwichedbetween ground plane layers, thereby forming signal line transmissionline layers which are isolated from one another in order to allow largescale integration. The signal line layers act as interconnect layers forinterconnecting active electronic components of a single electrical andstructural electronic module.

In a preferred MCM embodiment of the present invention there are twosignal line layers, an X layer and a Y layer, where X and Y indicatethat lines formed from each one of those layers extend generally alongone direction of the module and lines formed from the other layer extendalong the perpendicular direction. The X and Y signal line layers areonly seperated by a very thin and low dielectric constant dielectric sothat most of the energy of a travelling wave resides in the very lowdielectric constant MF₂ layer. Because superconducting signal lines maybe much narrower than non-superconducting signal lines a single set ofone X layer and one Y layer may replace a much larger number of normalmetallic non-superconducting signal line layers. Obvious reductions inmanufacturing and design costs and increases in operating speeds result.

FIG. 1A shows that type of structure in which substrate 1 is subdividedinto a plurality of five layers, 1A, 1B, 1C, 1D and 1E. Layer 1Ecomprises a substrate layer upon which is grown superconducting metaloxide layer 1D. The surface of layer 1E upon which superconducting metaloxide layer 1D is grown should be a material which is compatible withgrowth of the superconducting oxide, such as SrTiO₃. Layer 1C is alattice matching buffer layer which grows epitaxially upon thesuperconducting metal oxide layer 1D. For example, layer 1C may beSrTiO₃. Layer 1B is another superconducting metal oxide layer whichgrows epitaxially on lattice matching buffer layer 1C. Layer 1A is aalkaline earth fluoride layer upon which layer 2 shown in FIG. 1 is tobe grown.

Superconducting layers 1D and 1B may be used as power supply layers andground plane layers, respectively. Layer 1D is the power supply layerwhich is electrically isolated from pickup of high frequency signalsoccurring in high frequency lines above ground plane layer 1B because ofthe electrical isolation provided by ground plane layer 1B. Highfrequency signals may be transported via superconducting transmissionlines formed by forming signal lines from superconducting layer 4 inwhich case the transmission line is defined to include superconductingground plane line 1B, alkaline earth layer 1A and buffers layers 2 and3.

Layer 1E may comprise additional superconductor layers for signal linelayers and additional superconducting ground plane layers to isolate thesuperconducting signal line layers from one another. When more than onesuperconducting signal line layer exists those layers are preferablyisolated from one another by superconducting ground plane layers toreduce line cross talk and reduce line dispersion.

As with conventional semiconductor multilevel technology, via holes maybe used to provide interlayer contact. Ground plane layers may bereplaced by conducting materials which are not metal-oxides and whichare not superconducting since resistive losses associated with groundplane layers are relatively low.

SECOND EMBODIMENT

A more complete appreciation of the second embodiment of the inventionand many of the attendant advantages thereof will be readily obtained asthe same becomes better understood by reference to the followingdetailed description of the second embodiment when considered inconnection with the accompanying drawings, wherein FIG. 2 shows a crosssection of a layered structure sa comprising single crystal siliconsubstrate 5, buried oxide layer 6, upper silicon layer 7 buffer layer 8and metal oxide film 9.

Preferably silicon substrate 5 is oriented (100). Preferably, buriedoxide layer 6 is buried at a depth of about 2000 Å beneath the substratesurface, i.e., beneath the upper surface of upper silicon layer 7.

Buried oxide refers to formation of a silicon oxide layer byimplantation of oxygen into silicon and subsequent heating to formsilicon oxide. During that subsequent heating the implanted oxidecrystallizes with the adjacent silicon to form crystalline siliconoxide. The silicon oxide and adjacent upper silicon layer 7 phaseseparate and recrystallize during that process and form an atomicallysharp interface; between them.

Ion implantation provides a suitable method for doping a singlecrystalline silicon substrate in a layered region beneath a majorsurface of the substrate. During implantation of oxygen into a siliconsubstrate no surface-phase layers are created and only desired buriedoxide layers are formed. To activate oxygen atoms implanted into siliconand to remove implantation damage it is necessary to anneal a siliconsubstrate to a minimum temperature of 900° C. and that step is calledsolid phase epitaxy. During the step of solid state phase epitaxy, aburied oxide layer with well-controlled thickness is obtained leaving athin, single crystal Si layer above and an annealed Si substrate belowthe buried oxide layer. The interfaces between the Si and buried oxidelayer are atomically sharp. A more detailed description of the process,called the SIMOX process is described in a recent article by G. K.Celler and A. E. White in Materials Research Bulletin, June 1992. Awealth of information regarding silicon chemistry exists in theliterature as found in an article by C. A. Deckert in Thin FilmProcesses (Academic Press, Inc, 1978, p. 401) edited by J. L. Vossen,which is hereby incorporated by reference.

Upper silicon layer 7 is typically about 2000 Å thick and singlecrystal. Because of the atomically sharp interface formed between uppersilicon layer 7 and buried oxide layer 6 only a very small variation inthickness of upper silicon layer 7 exists. The thickness of uppersilicon layer 7 depends upon the oxygen implantation energy of oxygenimplanted during formation of buried layer 6.

Buffer layer 8 absorbs strain due to lattice constant differencesbetween upper silicon layer 7 and metal oxide film 9. Buffer layer 8also isolates metal oxide layer 9 when upper silicon layer 7 isdissolved with an etchant. Preferably, buffer layer 8 is oriented yttriastabilized zirconia oriented by its growth upon upper silicon layer 7.If upper silicon layer 7 is oriented (100) then buffer layer 8 will alsobe oriented (100). Preferably buffer layer 8 is between 100 and 5000 Å,and more preferably about 1000 Å thick.

Metal-oxide layer 9 is grown upon buffer layer S. In a preferredembodiment, metal oxide layer 9 is a superconducting thin film of YBCO,but may also be other metal oxide superconductors, pyroelectrics such as(BaSr)TiO₃, and ferroelectrics such as PZT, among others. Free standingfilms of those materials are useful at least as IR detectors.

The presence of a native surface oxide (SiO_(x)) on upper silicon layer7 reduces YSZ buffer layer epitaxy in the desired (100) orientation andoften leads to formation of YSZ in the undesirable (111) orientation. Itis therefore desirable to strip off the native top surface oxide andpassivate the surface by hydrogen termination before growing YSZ.Hydrogen termination is described by Fenner et al in Journal of AppliedPhysics, Volume 66, 419 (1989). The crystalline Si surface is hydrogenterminated if it is immersed in any of the HF reagents such as HF andwater or HF and ethanol.

After depositing metal oxide film 9 silicon substrate 5 is chemicallydissolved by a selective etchant while protecting metal oxide film 9from the etchant. Buried oxide 6 acts as an etch stop layer which is notetched by the selective etchant for silicon, preventing buffer layer 8and metal oxide layer 9 from being exposed to the selective etchant forsilicon.

After the substrate is dissolved away a thin free standing structureconsisting only of silicon oxide layer 6, silicon layer 7, buffer layer8, and metal oxide layer 9 remains. That structure has very low thermalmass due to its thinness so that metal-oxide layer 9 is therefore usefulin detector applications. The silicon oxide and YSZ layers may also beremoved by selective etching and/or ion beam etching, reactive ionetching, or reactive ion beam etching to leave only metal oxide layer 9as shown in FIG. 3. Metal oxide layer 9, by itself or with any of bufferlayer 8, silicon layer 7 and silicon oxide layer 6, may be attached to anew support backing 11 via glue 10 or its equivalent, as shown in FIG.4. While thin structures of metal-oxide layers have been described otherthin layer materials are useful, for example as membranes, and may bedeposited instead of metal oxide layer 9.

EXAMPLE 2

The invention was experimentally verified using pulsed laser ablationfor the deposition of both the buffer layers 4, as well as the YBCO filmS. This deposition method is well known in the deposition ofsuperconducting films and has been described in detail by Singh et al.in "Theoretical model for deposition of superconducting thin films usingpulsed laser evaporation technique," Journal of Applied Physics, Volume68, 1990, pp. 233-246.

In fabricating the sample of Example 2,the vacuum chamber was pumpeddown to vacuum levels of the order of 10⁻⁶ Torr, before letting theoxygen gas in to the chamber to maintain the required oxygen partialpressure during the deposition. All the three layers were deposited in asingle pump-down cycle, without exposing the chamber to the ambientatmosphere between each layer. The targets were held on a carrousel ofthe sort described by Chase et al. in "Multilayer high T_(c) filmstructures fabricated by pulsed laser deposition of Y--Ba--Cu--O",Journal of Materials Research, volume 4, 1989, pp. 1326-1329. During thedepositions, the carrousel would rotate the chosen target about thetarget axis with the laser beam hitting the circumference of the target.Between the depositions, the carrousel itself would rotate about its ownaxis to place the required target in the laser beam.

For our depositions, a KrF excimer laser delivered optical pulses of 248nm radiation with pulse widths of 20 ns, pulse energy densities of 1.5J/cm² on stoichiometric targets, and pulse repetition rates of 10 Hz.

A (100) Si substrate holder holding a substrate was held at atemperature of 725° C. during deposition of a YBCO film and at atemperature of 750° C. during deposition of a YSZ buffer layer.

The (100) Si substrate having a buried oxide layer was cut to dimensionsof 10 mm×10 mm and of 0.25 mm thick. It is commercially available fromIBIS Corp.

A YSZ buffer layer was deposited to a thickness of 1000 in an oxygenpartial pressure of 1 mTorr at 750° C. and found to be oriented (100).The target for the deposition of the (100) YSZ buffer layer was a singlecrystal of stoichiometric composition, which is available fromCommercial Crystal Labs, Naples, Fla.

The YBCO was deposited to a thickness of about 3000 Å. The target forthe laser deposition of the YBCO layer was a pellet of sintered powderof YBCO purchased from Seattle specialty ceramics of Woodinville, Wash.

After the depositions, the film of Example 2 was cooled to roomtemperature at the rate of 75° C. /min in 200 Torr of oxygen.

The superconducting transition temperature of the YBCO film of Example 2was determined by ac susceptibility and dc resistivity measurementscarried out as a function of temperature. The YBCO film exhibited asuperconducting transition temperature of 87K with a transition widthless than 1K.

The crystalline quality of the film-buffer layer combination of example2 was determined by x-ray diffraction. The diffraction studies indicateda high degree of crystalline quality with the c-axis of the film 5 thebuffer layers 4 and the substrate 1 all parallel to each other,indicating a high degree of epitaxial growth.

The Si substrate is etched away using an oxidation-reduction or "redox"etching process, which involves conversion of the material being etchedto a soluble higher oxidation state. Prior to etching, the YBCO filmsurface is protected with an epoxy resin available from Emerson andCuming Inc. of Woburn, Mass. This is done by warming the film surface toa temperature of 100° C. and applying a small bead of epoxy to thesurface. Two etching media presently used are (i) 33 wt % KOH solutionat 70° C., (ii) Ethylenediamine-pyrocatechol-water (EPW) solutions.Those media are used in conjunction with the immersion etching techniquewhereby a substrate to be etched is immersed in one of those etchants.

The substrate of Example 2 was immersion etched. The masked substrate(masked on the film side) was submerged in a liquid etchant. All theetching procedures were carried out in Teflon beakers since HF attacksglass. The etchant was mechanically agitated since that was found toimprove uniformity and control of the etching process. Further, one ofthe rate limiting steps of the etching process is the evolution of H₂gas bubbles. The Si substrate was etched at 5 gm/min until the substratewas entirely dissolved, leaving a free standing film comprising YBCO.

The free standing YBCO film was then epoxied onto a single crystallinediamond substrate with a thin layer of epoxy (GE 1052). The film withthe epoxy and diamond substrate was air dried under a tungsten lamp for24 hours. The resin protecting the YBCO film was washed away in acetoneexposing the YBCO film.

The superconducting film was determined to have a transition temperatureabove 77K.

Although the invention has been described with reference to YBCO, almostall of the high T_(c) oxide superconductors have crystal structuressimilar to that of YBCO and exhibit similar superconducting properties,for example the bismuth and thallium cuprate superconductors and bismuthpotassium oxide superconductors. Therefore those materials can besubstituted for the YBCO of Example 2. Although pulsed laser depositionwas used in Example 2, the invention may be practiced with all otherdeposition techniques, e.g., magnetron sputtering and e-beam deposition.

Although the invention has been described with reference tosuperconducting oxides, it is applicable to any epitaxial oxideover-layer such as ferroelectric oxides and photorefractive oxides.

A buried oxide layer provides an atomically flat surface at whichetching of a selctive etchant stops. Since the buried oxide interface isatomically flat, very small variations in thickness of the slectivelyetched free standing structure are obtained. Variations in thicknessacross a 1 cm by 1 cm sample of less than 500 Å may be achieved. Becauseburied oxides may be uniformly prepared to have a uniform depth in thesubstrate across a large wafer, such as a 3 inch diameter wafer, andbecause the buried oxide provides an atomically sharp etch stop,membranes having an average thickness as thin as 1000 Å may be providedacross an entire 3 inch diameter wafer by the process disclosed above.

Although the invention has been described with reference to a Sisubstrate with a buried oxide, it also is clear to one of ordinary skillto apply to other substrates with a buried oxide. Likewise other buriedlayers besides oxides may be used so long as they form, uponrecrystallization, an atomically sharp interface between the buriedlayer and the recrystallized single crystal layer 7.

THIRD EMBODIMENT

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description of thethird embodiment when considered in connection with the accompanyingdrawings, wherein FIG. 5 shows a cross-section of an overview of asubstrate fixture 21 connected via support rods 14 and rotary power feed13 to flange 12. Substrate fixture 21 is preferably used in a lowpressure vessel and flange 12 is used to connect and hold substratefixture 21 inside that vacuum vessel. Support rod 14 is coupled viarotary power coupler 15 to rotary shafts 34, 34a as shown in FIG. 6.

Although rotary shafts 34, 34a, are shown to provide power along theaxis of the substrate magazine, it is equally possible for rotary shafts34, 34a to provide rotary power along a peripheral edge of substratemagazine 33.

Rotary shafts 34, 34a are preferably restrained to rotary movement byball bearings 16 and ball bearing support housing 17 which is connectedto flange 12 via support rods 14. Support arms 18a, 18b connect to ballbearing support housing 17 and to substrate fixture 21, thereby holdingsubstrate fixture 21 motionless.

The overview of FIG. 5 also shows the orientation of substrate fixture21 to a PLD target 19 and plume 20 which is generated when a PLD pulseimpinges upon target 19. The arrow shown in plume 20 indicates theaverage momentum vector of plume 20 which is directed toward substratefixture 21. Substrate fixture 21 contains substrate magazine 22.Substrate magazine 22 provides means for simultaneously holding aplurality of substrates in substrate fixture 21 and will be discussed inmore detail below.

FIGS. 5A and 5B show alternate embodiments of the substrate fixture 21of FIG. 5 in which laser plume 20 scans along the rotation axes of thesubstrates in order to provide uniform coating to the subsstrates. FIG.5A showsextension 101 of ball bearing support housing 17 slidablysupporting mask support rods 101A. Mask support rods 101A hold mask 100between substrate fixture 21 and target 19 so that plume 20 must passthrough aperture 28A of mask 100. Motor 103 drives the lower masksupport rod 101A via support 102.

During operation of the embodiment of FIG. 5A mask 100 and the point atwhich the laser beam impinges target 19 are simultaneously scanned alongthe rotation axes of the substrates in order to uniformly coat the stackof substrates. Mask 100 is narrower than aperture 28 and provides heatshielding in order to maintain uniform temperature to the substrates.

FIG. 5B shows an embodiment where the point of ablation relative to thecontainment chamber containing the substrate fixture does not vary withtime, but the sbustrates inside fixture housing 27 are translated alongtheir axes of rotation to scan them past aperture 28 in order to provideuniform coating to the substrates. Gears 106 and 107 translate rotarypower provided by an external motor (not shown) external to thecontainment chamber into the cahmber and to threaded rod 105. Rotationof threaded rod 105 drives rotary motor 104 along its axes of rotationthereby translating the substrates. Preferably, axial length 108 of theinside of fixture housing 27 is at least twice as long as axial length109 of substrate magazine 33 so that substrate magazine 33 may betranslated to provide uniform coating to all the substrates within it.

FIG. 6 shows a more detailed view of substrate fixture 21 of FIG. 5which shows thermocouple 23 extending into substrate fixture 21 throughfixture housing upper half 27b. Fixture housing upper half 27binterconnects with fixture housing lower half 27a to form an enclosuresubstantially enclosing substrate magazine 22. Gas lines 24 provide aflow of gas at a pressure above the background pressure in the pressurevessel to substrate fixture 21. More specifically, gas nozzles 24provide gas to that side of substrate fixture 21 opposing target 19 andgas nozzles 25 are directed so that gas exiting gas nozzle 25 has anaverage momentum component in the same direction as the net momentumdirection of the plume. In other words, the nozzles point generally awayfrom target 19 and enter either fixture housing lower half 27a orfixture housing upper half 27b along the side of the housing facingtarget 19 and direct gas through substrate fixture 21 toward a side ofsubstrate fixture 21 which is furthest from target 19.

Heater power wires 26 provide power to heater blocks 30, 31. Heaterblocks 30, 31 are connected to the stationary support arms 18a, 18b. Ina presently preferred embodiment the heated area of each opposing heaterblock 30, 31 is between 3 and 20 square inches. Radiant energy iscontained within the heater assembly by radiation reflecting/shielding(not shown) creating a "blackbody" affect.

The fixture housing consisting of fixture housing lower half 27a andfixture housing upper half 27b includes several openings, including gasescape holes 29 disposed on that side of substrate fixture 21 which isaway from target 19 and fixture housing front side opening 28 which isan opening on the side of substrate fixture 21 that faces target 19 andthrough which material from plume 20 enters into the housing ofsubstrate fixture 21. Gas nozzles 25 may pass through fixture housingfront side opening 28 or may pass through additional openings in thesubstrate fixture housing.

Substrate magazine 33 inside substrate fixture 21 is connected along itsvertical axis to rotary shafts 34, 34a. During operation substratemagazine 33 holding substrates 32 is rotated via power supplied byrotary shafts 34, 34a. Substrates 32 have a radius R and preferably areseparated by less than R. As discussed below, the separation distancebetween substrates affects deposition thickness uniformity. Also, asillustrated by Examples 4 and 5 below, when substrates are separated byat least R/4 which is 1/4 inch in those examples, material is depositedover the center of a two inch diameter substrate. Examples 4 and 5indicate that when substrates are separated by at least 1/4 inch or R/4and rotated during deposition an entire 2 inch substrate is coated.

Substrates 32 are held in position in substrate magazine 33 by groovesin rods 39 into which edges of the substrates fit.

FIGS. 7a-7c show a more detailed view of substrate magazine 32 and howsubstrate magazine 32 is connectable and detachable from rotary shafts34, 34a. FIG. 7a shows a plan view of substrate magazine 33 in whichfour vertical rods 39, also shown in FIG. 7b, hold upper and lowerradial supports 40, 40a, at a fixed separation. Upper and lower radialsupports 40, 40a are connected to rotary shafts 34, 34a, respectively,as shown in FIG. 7c. Rotary shafts 34, 34a, are vertically translatableso that they may be withdrawn from substrate magazine 33 so thatsubstrate magazine 33 may be horizontally displaced and removed fromsubstrate fixture 21. Rotary shafts 34, 34a, may be vertically displacedby sliding them along sleeves 37, 37a, of translatable rotary shaftconnectors 35, 35a. Rotary shafts, 34, 34a, may be secured againstfurther vertical translation by insertion of screws 36 as shown in FIG.7c.

Substrate fixture 21 is operated by connecting flange 12 to the wall ofa vacuum chamber. Substrate magazine 33 is assembled as shown in FIG. 7bwith substrates 32 held in position by notches spaced along the lengthof rods 39. Rods 39 are secured to upper and lower radial supports 40,40a via screws (not shown). Substrate magazine 33 is then disposedinside substrate fixture 21. Rotary shafts 34, 34a are extended untilthey connect with substrate fixture 33 and rotary shafts 34, 34a, arethen secured in position by screws 36. Next, the vacuum vesselcontaining substrate fixture 21 is pumped down to a pressure determineduseful for PLD. Substrate fixture 21 is heated by supplying power toheater power wires 26 to heater blocks 30, 31 and temperature ismonitored and controlled using thermocouple 23. Substrate magazine 22 isrotated by rotary power supplied via rotary shafts 34, 34a andpreferably at between 1 and 1000 revolutions per minute. A supply of gasis provided through gas nozzles 25.

It is important to note that gas supplied through gas nozzles 25 ispreferably supplied so that a flow of gas is generated from the frontside of substrate fixture 21, i.e., the side of substrate fixture 21facing target 19, along the surfaces of the substrates and toward gasescape holes 29 at the back side of substrate fixture 21 through whichthe supplied gas escapes from substrate fixture 21. Preferably, chamberpressure is maintained constant through use of a vacuum pumping system(not shown). The flow of supplied gas along the momentum direction ofplume 20 is believed to break up an acoustic standing wave patterngenerated by supersonic flow of plume 20 through substrate magazine 33.As discussed below, that supersonic flow and the acoustic standing wavepattern set up thereby is believed to cause inhomogeneous thickness ofmaterial deposited on substrates 32, and that inhomogeneity isalleviated by flowing gas along the momentum direction of plume 20momentum.

Once thermocouple 23 indicates an appropriate temperature for depositionand gas flow has been initiated PLD is commenced by impinging target 19with high fluence laser pulses. Those pulses generate plume 20, amajority of which passes through fixture housing front side opening 28,travelling roughly parallel to the planes defined by substrates 32.

Substrate fixture 21 is aligned so that substrates 32 have their majorsurfaces perpendicular to the normal for target surface 19. Thatalignment is preferably within ±30° and more Preferably within ±10° foreach of the substrates in order to provide adequate coating of bothsides of substrates 32 and to avoid deposition of submicron sizeparticles ejected from the surface of target 19 during ablation, whichare carried along with the plume into substrate fixture 21, ontosubstrates 32.

Alternatively, if deposition onto only a single substrate side isdesired, planes defined by the substrate surfaces may be canted at anangle relative to the target surface normal thereby preferentiallyexposing one side of each substrate to the plume, but still enablingmultiple substrates 32 to be simultaneously coated. After a desirednumber of PLD laser pulses, deposition is discontinued, substrates 32are cooled and eventually removed from substrate fixture 21 and fromsubstrate magazine 33.

Preferably, substrates 32 are spaced very close together in order toprovide as many substrates as possible so that an angle between thesubstrate normal and vertical displacement from the substrate normal tothe center of any of substrates 32 and magazine 33 is less than 30° .Typically target 19 is within 12 inches and more preferably within 6inches of substrate fixture 21. Opening 28 preferably is no larger thannecessary to allow that part of the plume which is within ±30° of theplane defined by the target surface normal and the substrate surfaces toenter substrate fixture 21. Therefore a relationship exists betweenopening 28 and the distance from target 19 to the front side ofsubstrate fixture 21 limiting the total width of opening 28perpendicular to the substrate surfaces.

In any case it is preferable to provide as many substrates at one timeas possible in substrate fixture 21. Therefore, it is preferable toprovide those substrates with a separation of less than 5 mm betweenthem and more preferably with a separation of less than 2 mm betweenthem. Preferably, substrate magazine 33 can hold at least threesubstrates at one time and more preferably at least seven substrates atone time. Increasing the number of substrates increases the yield perPLD deposition and also increases the deposition uniformity for centralsubstrates, because substrates at the upper and lower ends of a stack insubstrate magazine 33 have their outer sides directly exposed tosubstrate heater blocks 31, 32. Substrate magazine 33 holds substratesgenerally parallel to one another, i.e., within a few degrees fromparallel.

Experimental Examples 3-5 which are discussed with reference to FIGS.8a-8c are provided below.

EXAMPLE 3

FIG. 8a shows substrate 43 on flat plate heater 42. Flat plate heater 42is thermally isolated from support rod 44 by ceramic standoffs 45.Substrate 43 has its major surface aligned with a normal from thesurface of target 19 so that plume 2b has its momentum direction in theplane of the major surface of substrate 43. Using the configurationshown in FIG. 8a substrate 43 was coated by PLD from target 19. Target19 was YBCO and substrate 43 was LaAlO₃. Deposition rate onto substrate43 shown in FIG. 8a was found to be within 30% of the deposition ratethat occurs when the surface of a substrate to be coated faces plume 20.However, no submicron particles were found to exist in the coatingprepared on substrate 43. The coating on substrate 43 was found to havesuperconducting properties similar to properties for films prepared whena major surface of a substrate is oriented to face plume 20.

EXAMPLE 4

FIG. 8b shows the configuration for Example 4 in which substrates 46were separated by spacer 47 from one another and heated by flat plateheater 42. Flat plate heater 42 was thermally isolated from support rods44 by ceramic standoffs 45. Thickness of substrates 46 ranged between0.001 and 0.050 inches. Again, plume 20 had a momentum direction in theplane of the major surfaces of substrates 46. The substrates were 2 inchdiameter silicon wafers.

In Example 4, substrates 46 were separated by 0.125 inches by spacers47.

FIG. 8c shows one of the opposing sides of one of the substrates 46 inwhich the momentum direction of the plume is indicated. That substrateshows a pattern indicating thickness ridges 47. Between each two linesshown on the substrate 46 in FIG. 8c is a region of constant thickness.Several separate regions are shown. Those regions do not quite encompassthe center of substrate 46.

EXAMPLE 5

The same configuration as shown in Example 4 was used, the onlydifference being that spacers 47 separated substrates 46 by 0.25 inches.In Example 5 thickness ridges similar to thickness ridges 47 shown inFIG. 8c occurred, but those ridges extended to encompass the center ofsubstrate 46. In both Examples 4 and 5 target 19 was between 2 and 5inches from the closest portion of the substrates.

The ridges shown in FIG. 8c are due to constrained flow dynamicsoccurring in the cavity defined by the two opposing faces of thesubstrates. That constrained flow dynamics sets up static pressure wavesbetween the substrates that result in regions of deposited film ofdifferent thickness. Flowing gas along the surface of the substrates ina direction along which the plume travels removes the standing wavepattern resulting in a more uniform deposition. Additionally, rotationof the substrates during deposition circularly homogenizes the ridges 47shown in FIG. 8c.

Obviously numerous modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionsmay be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A superconducting device, comprising:asubstrate having a lower surface and comprising MF₂ and having an MF₂surface that is above the substrate lower surface where M is selectedfrom a member of the group consisting of Be, Mg, Ca, Sr and Ba; a firstisolation buffer layer comprising MgO that is disposed on and inepitaxial contact with the substrate MF₂ surface; a second isolation andlattice matching buffer layer disposed on and in contact with the firstbuffer layer where said second isolation and lattice matching bufferlayer is a perovskite crystal structure material and is selected from amember of the group consisting of NdGaO₃ and SrTiO₃ ; and asuperconducting metal-oxide layer disposed on and in epitaxial contactwith the second buffer layer where said superconducting metal-oxidelayer is selected from a member of the group consisting of Y₁ Ba₂ Cu₃O_(7-x) where x is equal to or greater than zero and is less than orequal to one-half, TlBaCaCuO and BiSrCaCuO.
 2. A superconducting deviceaccording to claim 1 wherein:M consists essentially of Mg.
 3. Asuperconducting device according to claim 1 wherein:the MF₂ substratecomprises a single crystal of MF₂.
 4. A superconducting device accordingto claim 1 wherein:the first isolation buffer layer has a thickness thatis between 30 angstroms and one micron.
 5. The superconducting deviceaccording to claim 1 wherein the second isolation and lattice matchingbuffer layer is in epitaxial contact with the first isolation bufferlayer.
 6. The superconducting device according to claim 1 furthercomprising a third buffer layer between the first and second bufferlayers, wherein the third buffer layer is in epitaxial contact with thefirst and second buffer layers.
 7. A superconducting device,comprising:a substrate having a lower surface and comprising MF₂ andhaving an MF₂ surface that is above the substrate lower surface where Mis selected from a member of the group consisting of Be, Mg, Ca, Sr andBa; a first isolation buffer layer comprising MgO that is disposed onand in epitaxial contact with the substrate MF₂ surface; a secondisolation and lattice matching buffer layer disposed on and in contactwith the first buffer layer where said second isolation and latticematching buffer layer is selected from a member of the group consistingof CeO₂, yttria stabilized zirconia, NdCaAlO₄, LaSrAlO₄, and LaSrGaO₄ ;and a superconducting metal-oxide layer disposed on and in epitaxialcontact with the second buffer layer where said superconductingmetal-oxide layer is selected from a member of the group consisting ofY₁ Ba₂ Cu₃ O_(7-x) where x is equal to or greater than zero and is lessthan or equal to one-half, TlBaCaCuO, and BiSrCaCuO.
 8. Thesuperconducting device as recited in claim 7 where M consistsessentially of Mg.
 9. The superconducting device as recited in claim 7wherein:the MF₂ substrate comprises a single crystal of MF₂.
 10. Thesuperconducting device as recited in claim 7 wherein:the first isolationbuffer layer has a thickness that is between 30 angstroms and onemicron.
 11. The superconducting device as recited in claim 7 wherein thesecond isolation and lattice matching buffer layer is in epitaxialcontact with the first isolation buffer layer.
 12. The superconductingdevice as recited in claim 7 further comprising a third buffer layerbetween the first and second buffer layers, wherein the third bufferlayer is in epitaxial contact with the first and second buffer layers.